Stabilized electroless plating solutions

ABSTRACT

An electroless plating solution is characterized by the addition of a small but effective amount of a covalent mercury compound to increase bath stability. It is known in the art that solutions for electroless plating are unstable and tend to decompose with use. It is also known that decomposition can be retarded and the useful life of a plating solution increased by the addition of various additives, frequently catalytic poisons, in very small concentrations. In accordance with the present invention, it has been found that the stability of an electroless plating solution can be substantially increased by the addition of a covalent mercury compound alone as a primary stabilizer, or preferably, by the addition of the mercury compound as a secondary stabilizer in combination with a prior art stabilizer as primary stabilizer. The combination of stabilizers provides a synergism with stability substantially improved over that obtainable with either component of the combination alone.

United States Gulla et al.

atent [54] STABILIZED ELECTROLESS PLATING SOLUTIONS [72] Inventors: Michael Gulla, Newton; Oleh B. Dutkwych, Meefield, both of Mass.

[73] Assignee: Shipley Company, Inc., Newton, Mass.

221 Filed: May 21, 1970 [21] Appl. No.: 39,503

Related US. Application Data [63] Continuation-in-part of Ser. No. 785,350, Dec. 19,

[56] References Cited UNITED STATES PATENTS 3,295,999 l/1967 Kleinetal ..l06/1 [451 Mar. 14, 1972 Primary Examiner-Lorenzo B. Hayes Attorney-Roberts, Cushman and Grover [5 7] ABSTRACT An electroless plating solution is characterized by the addition of a small but effective amount of a covalent mercury compound to increase bath stability. It is known in the art that solutions for electroless plating are unstable and tend to decompose with use. It is also known that decomposition can be retarded and the useful life of a plating solution increased by the addition of various additives, frequently catalytic poisons, in very small concentrations. In accordance with the present invention, it has been found that the stability of an electroless plating solution can be substantially increased by the addition of a covalent mercury compound alone as a primary stabilizer, or preferably, by the addition of the mercury compound as a secondary stabilizer in combination with a prior art stabilizer as primary stabilizer. The combination of stabilizers provides a synergism with stability substantially improved over that obtainable with either component of the combination alone.

20 Claims, No Drawings STABILIZED ELECTROLESS PLATING SOLUTIONS CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of copending U.S. Pat. application Ser. No. 785,350, filed Dec. 19, 1968.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a metal-depositing composition and more particularly, to an electroless metal plating solution having extended life and temperature tolerance and characterized by the addition of a stabilizer comprising a covalent mercury compound alone or in combination with a prior art stabilizer.

2. Description of the Prior Art Electroless metal deposition refers to the chemical plating of a metal over an active surface by chemical reduction in the absence of an external electric current. Processes and compositions useful therefor are known, are in substantial commercial use, and are described in numerous publications. For example. compositions for depositing electroless copper are described in U.S. Pat. Nos. 2,938,805; 3,011,920; and 3,383,224 and compositions for depositing electroless nickel are described in U.S. Pat. Nos. 2,690,401; 2,690,402;

2,762,723; 2,935,425; 2,929,742; and 3,338,726.

Known electroless metal deposition solutions generally comprise at least four ingredients dissolved in a solvent, usually water. They are (1) a source of the metal ions, (2) a reducing agent such as formaldehyde for copper or hypophosphite for nickel, (3) an acid or hydroxide pH adjuster to provide required pH, and (4) a complexing agent for metal ions sufficient to prevent their precipitation in solution. A large number of suitable complexing ions for electroless metal solutions are described in the above noted publications and also in U.S. Pat. Nos. 2,874,072; 3,075,856; and 8,075,855 also incorporated herein by reference. In some formulations, the complexing agent is helpful but not a necessity.

Although electroless metal solutions have been used for many years, the commercially used formulations have not been fully satisfactory for several reasons. Among these are relatively slow deposition rates and bath instability. It has been shown that plating rate is dependent to some extent, upon the concentration of the reducing agent in the plating solution and that increased concentration will generally result in an increased rate of deposition. However, increased concentration of reducing agent also results in decreased bath stability. This is evidenced by a decrease in the time in which the plating solution will undergo uncontrollable decomposition (trigger).

lt is known in the art that certain additives or inhibitors added to an electroless metal solution in properly controlled trace quantities act as stabilizers and retard the rate of bath decomposition. Generally speaking, these additives, or stabilizers as they are referred to in the art, are catalytic poisons. The concentration of the stabilizer in solution is usually critical. Trace quantities, typically in the range of a few parts per million, provide stability. An excess of stabilizer will partially or totally stop deposition of the electroless metal.

In the art of electroless metal deposition, it is known that stabilizing agents useful for stabilizing a solution of one metal may or may not be useful for the stabilization of a solution of a different metal. For example, thio compounds such as thiourea and alkali thiosulfates are useful stabilizers for both nickel and copper solutions while cyanide compounds are used only for the stabilization of copper solutions.

In copending U.S. Pat. application Ser. No. 785,350, filed Dec. 19, 1968, there is disclosed an improved stabilizing system for electroless plating solution. This invention is predicated upon the discovery that the addition ofa small but effective amount of a source of mercury ions to substantially any electroless metal solution improves stability without retarding the rate of deposition and in some solutions, provides a somewhat improved rate of deposition. Moreover, in accordance with this invention, it was found that the addition of a combination of the mercury ions with a prior art stabilizer provided a synergism resulting in a substantially increased solution stability.

STATEMENT OF THE INVENTION This invention is predicated upon the discovery that covalent mercury compounds are as effective as mercury ions as stabilizers for electroless solutions, and further, may also be used in combination with prior art stabilizers to provide a synergism resulting in substantially increased solution stability. Accordingly the present invention provides an electroless metal solution comprising: l) a source of metal ions, (2) a reducing agent therefor such as formaldehyde for copper or hypophosphite for nickel, (3) a pH adjuster, (4) a complexing agent for the metal ions sufficient to prevent their precipitation in solution where necessary, and (5) a stabilizer for the solution which may be a covalent mercury compound alone, as a primary stabilizer or in combination with a prior art stabilizer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As noted above, the covalent mercury compound is believed to be responsible for increased bath stability. For purposes of this invention, a covalent mercury compound is defined as one that does not dissociate in an electroless plating solution to yield mercury ions (Hg**). The mercury compound contemplated may be represented by the formula R- HgR where R represents a radical such as alkyl including cyloalkyl, aryl, alkaryl, aralkyl, alkoxy, aryloxy, heterocyclic and the like, and R represents the same radicals as R and in addition polar groups such as NO SO OH, and metal salts such as sodium salts, COOH and metal salts such as sodium salts, NH halides such as Cl, Br and l, CN, and the like. Examples of compounds corresponding to the above formula include ethylmercuric hydroxide, ethylmercuric iodide, mercury ethylmercaptide (ic), mercury phenylmercaptide (ic), methylmercuric chloride, methylmercuric iodide, diisopropylmercury, dimethylanilinemercury (p), dimethylmercury, dinaphthylmercury (a), dinaphthylmercury (B), dipropylmercury, ditolylmercury (o), ditolylmercury (m), ditolylmercury (p), phenylmercuric bromide, phenylmercuric chloride, phenylmercuric cyanide, phenylmercuric iodide, phenylmercuric nitrate, tolylmercuric bromide (p), biphenylmercury, chloromercuriphenol (o), di-n-amylmercury, di-(dl)-amylmercury, dibenzylmercury, di-n-butylmercury, di-n-hexylmercury, diisoamylmercury, diidobutylmercury, the sodium salt of mercuriphenoldisulphamite, the sodium salt of 2,4-dihydroxy-3,5-di(hydroxymercuri) benzophenone-2'- sulphonate, the sodium salt of 0-[(3-hydroxymercuric-2- methoxypropyl)carbonyl] phenoxyacetic acid and the like.

Preferably, the covalent mercury compound contains polar groups either attached to the radical R or as represented by R. These polar groups enhance the solubility of the covalent mercury compounds in solution. Preferred polar groups are OH and alkali metal salts of COOH and SO OH.

It should be noted that many of the above-notedcovalent mercury compounds are only poorly soluble in aqueous solutions and many are considered insoluble. However, the mercury compound, for purposes of the present invention, is required in amounts of only parts per million. Consequently, mercury compounds considered insoluble in aqueous solution may be soluble to the extent that they yield mercury ions in concentrations sufficient for purposes of the present invention. Mercury compounds soluble in aqueous solution are preferred.

Since the covalent mercury compounds are not believed to be catalytic poisons, their concentration in solution is not critical. Frequently, trace quantities are suitable. A preferred range comprises from about l to parts per million. However, amounts up to saturation in solution may be used or amounts in excess of saturation may be used, the excess serving as a source of the mercury compound for replenishment.

In a preferred embodiment of this invention, the covalent mercury compound is used as a stabilizer in combination with a prior art stabilizer. Materials known to the art as catalytic poisons to the deposition of electroless metal are frequently used in controlled amounts as stabilizers for electroless plating solutions. Perhaps the most widely used group of compounds of this nature are the divalent sulphur-containing compounds, many ofwhich are disclosed in U.S. Pat. No. 3,361,540, incorporated herein by reference. Representative examples of such sulphur compounds are the inorganic sulfides such as sodium sulfide, potassium sulfide, sodium polysulfide, and potassium polysulfide; organic and inorganic thio compounds such as sodium thiocyanate, potassium thiocyanate, potassium dithionate, sodium thiosulfate, and potassium thiosulfate; and organic sulphur containing compounds such as thiourea, 2- mercaptobenzothiazole, l,2-ethanedithiol, 1,2- benziosothiazane, methionine, 2,2'thiodiethanol, dithioglycol, and thioglycollic acid.

The amount of the sulfur compound used in combination with the mercury compound is small and will vary depending upon the particular compound used. Typically, the sulfur compound is present in an amount less than that which will not stop deposition of the plating metal. Generally, the amount may vary from a trace to about 300 parts per million dependent upon the sulfur compound used.

There are many stabilizers in the prior art in addition to divalent sulfur compounds. One other class of stabilizers comprises the water-soluble cyanide compounds defined broadly to include nitriles and dinitriles set forth in U.S. Pat. No. 3,3l0,430. Typical of such compounds are alkali metal cyanides such as sodium and potassium cyanide; nitriles such as alpha-hydroxynitrile, e.g., glyconitrile and lactonitrile, and dinitriles such as im inodiacetonitrile and 3 ,3 iminodipropionitrile. The cyanide compound is used in an amount about equal to that of the divalent sulfur compound.

A class of stabilizers for electroless copper solutions is disclosed in U.S. Pat. No. 3,457,089, tiled in the names of Charles Shipley and Michael Gulla. These stabilizers are acetylinic compounds corresponding to one of the following generic formulas:

R-C 5 CH or RC E CR where each R is individually selected from the class of lower monovalent hydroxyalkyl, cyclohydroxyalkyl or hydroxyalkyl ether. Examples include ethynyl cyclohexanol, methyl butynol, methyl pentynol, dimethyl hexynol, 2-butyne-l,4-diol, dimethyl hexynediol, propargyl alcohol, hexynol and ethyl octynol.

It has been found that the above class of compounds can be broadened to include substantially more members when used in combination with a mercury compound and can be used in electroless solutions in addition to copper. For example, when used in combination with a mercury compound, each of R,R and R" can be aryl or aliphatic including cycloaliphatic substituted with a watersolubilizing group such as hydroxyl and carboxyl. For the purposes of the present invention, the acetylinic compounds contemplated will be referred to by the term solution soluble acetylinic compounds.

iodate compounds such as sodium and potassium iodate and Pb ions are useful stabilizers for electroless nickel solutions. Both iodate and Pb ions are used in amounts of a few parts per million, preferably less than 50 parts per million. Additional examples of metals useful as stabilizers for electroless copper solutions are disclosed in the aforementioned U.S. Pat. No. 3,310,430.

' An electroless plating solution stabilized with a covalent mercury compound in accordance with this invention is used to deposit metal in the same manner as any prior art electroless solution. The surface of the part to be plated should be free of grease and contaminating material. Where a nonmetallic surface is to be plated, the surface area to receive the deposit must first be sensitized to render it catalytic to the reception of the electroless metal as by the well-known treatment with an acidic aqueous solution of stannous chloride followed by treatment with dilute aqueous acidic solution of palladium chloride. Alternatively, extremely good sensitization of nonmetallic surfaces is achieved by contact with a colloid of a precious metal having a protective stannic acid colloid and formed by the admixture of stannous chloride and a precious metal chloride, preferably palladium chloride, the stannous chloride being present in stoichiometric excess based upon the amount ofprecious metal chloride.

The invention will be better understood by reference to the following examples where stability of solution was measured by the time it takes for a bath to spontaneously decompose (trigger) when plating a catalyzed cloth at one-half square foot per gallon. In all examples, catalyzed cloth was prepared by treating a cotton fabric according to the following sequence of steps:

1. Rinse cloth in a 20 percent (by weight) ammonium hydroxide solution maintained at room temperature for 5 minutes. Rinse in cold water.

2. Rinse for 5 minutes in 20 percent acetic acid solution maintained at room temperature. Rinse in cold water.

3. Immerse for from 20 to 40 seconds in a sensitizing solution of a palladium colloid having a protective stannic acid colloid (Catalyst 6F) maintained at room temperature. Rinse in cold water.

4. Immerse for l to 3 minutes in a dilute hydrochloric acid solution maintained at room temperature. Rinse in cold water.

5. Dry cloth and cut to size.

In the following examples where rate of metal deposition from solution is recorded, rate was determined by plating over a phenolic substrate using the following procedure:

I. Cut phenolic board to a size measuring 2 inches by 2 inches.

2. Scrub clean with an abrasive cleaner. Rinse in cold water.

3. Treat for from 1 to 3 minutes with a nonionic surfactant conditioner maintained at room temperature. Rinse in cold water.

4. Immerse for from 3 to 4 minutes in a sensitizing solution of colloidal palladium having a stannic acid protective colloid (Catalyst 6F) maintained at room temperature. Rinse in cold water.

5. Immerse for from 3 to 6 minutes in dilute hydrochloric acid solution maintained at room temperature. Rinse in cold water.

6. Deposit electroless metal for a period of IO minutes.

7. Rinse, dry parts and measure thickness of deposit. in the examples, Example number and Bath number are used interchangeably.

EXAMPLES l to 10 Cupric sulfate peritahydrate 8 g. Paraformaldehydc 7.5 g. Tetrahydroxypro ylethylenediamine 12 g. Triisopropanolamine 2 g. Sodium hydroxide (28 wt /z] 50 ml. Water to I liter Temperature l205 F.

Catalyzed cloth was plated with the above formulation with stabilizing additives added in amounts and with results as set forth in the following table:

STABILiZING AGENTlS) (P-P:m.)

acetate (20 5 methyl butynol ii-2 (50) 6 henylmercuric methyl butynol 60 acetate (20) (50) 7 thiomalic acid 20-25 i phenylmercuric thiomalic acid 60 ucetutelZO) (l5) 9 mersalyl 60 10 mersalyl (30) meth)l butynol 60 sodium salt of ()-[(3-hydroxymercuri-Z-methoxypropyl)carbomyl] phenox yacctic acld From the above table, it is apparent that the addition of the mercury compound in small quantities to the copper deposition solution substantially extends the time for solution decomposition and provides a solution more stable than that possible with other prior art stabilizers for copper solutions. In addition, the combination of a mercury compound with the prior art stabilizers provides a synergism with solution stability extended to a time in excess of that using either stabilizer alone.

EXAMPLES 11-14 Cuprtc sulfate pentahydrate 8 g. Puraformuldehyde 7.5 g Rochelle salts 40 g NuOHlZMvtL'il 50g. Writer to l liter Temperature 120:5" F.

A sodiunvpotus ium turtrute double salt.

Catalyzed cloth is plated by immersion in the above solution containing stabilizers in amounts and with results as set forth in the following table:

STABILIZING AGENT (p.p.m.)

Bath Time to Trigger No Mercury Other (mini) l l ''l 12 phenylmercuric 2-3 acetate (20 I NuCN( 5) 3-4 l4 phenylmercuric NaCNtS) 8-9 acetate (1 EXAMPLES l5-18 Cu ric sulfate pentahydrate 8 gr Puruformaldehyde 7.5 g Pentah droxypropyidiethylenc- 20 g. triamine Sodium hydroxide (28 wt. .1?) 50 ml uter to 1 liter Temperature 120":5" F.

Catalyzed cloth was plated by immersion in the above formulation with stabilizers added in amounts and with results as set forth in the following table:

STABlLlZlNG AGENT(S) (p.p.m.)

Bath Time to Trigger No. Mercury Other (min 1 l5 :4 l6 phenylmercuric 3-5 acetate (41)) I? methyl butynol 2-2 (50) I8 phcnylmercuric methyl butynol 60 acetate (40) (50) The synergism between the mercury compound and the prior art stabilizers is readily shown by reference to the above table where using either stabilizer alone provided a plating solution that triggered within a few minutes. Combining the stabilizers (Bath No. 18) provided a bath that did not decom pose within 60 minutes.

EXAMPLES 19-24 Using the bath formulation of Examples l-8 at a temperature of 75+ F., the effect of the addition of stabilizers on plating rate was determined by plating a phenolic substrate in the manner noted above. The bath formulations used and plating rate are set forth in the following table:

Bath No.

The addition of mercuric acetate alone to the plating solution (Bath 2) results in some slight decrease in plating rate. The addition of methyl butynol (Bath 5) causes a somewhat greater decrease in rate. However, the addition of thiomalic acid to the plating solution (Bath 7) results in a substantial decrease in rate. Combination of mercuric acetate with the thiomalic acid (Bath 8) does not substantially further decrease the rate.

EXAMPLES 25-30 It is known in the art that contaminants dragged into an electroless plating solution result in a substantial decrease in bath stability. To determine the effect of contamination of an electroless solution containing a mercury compound, various contaminants known to accelerate triggering of an electroless solution were added to baths Nos. 7 and 8 in amounts and with results as set forth in the following table:

Bath Time to Trigger Nor Contaminant (mint) 7 60 8 60 7 PVP/VA 535 20-25 8 PVP/VA 535 60 7 dextrose" l0-12 8 dextrose 25-30 Polyvinylpyrrolidonewinyl acetate copolymer added in an amount ofSO p.pimt

Added in an amount ofone-hulfgrami EXAMPLES 31-34 It is known in the art that increased bath temperature results in a more rapid decomposition of an electroless plating solution To determine the effect of temperature on the plating solutions of this invention containing covalent mercury compounds alone or in combination with other stabilizers, Baths Nos. 1, 2, 7 and 8 were used to plate catalyzed cloth at varying temperatures with results as set forth in the following table:

Bath No. 8 illustrates the substantial improvement and the synergism resulting from the combination of the covalent mercury compound with a prior art stabilizing compound.

EXAMPLES 35SO Nickel sulfate hcxahydratc 35 g. Sodium hypophospite mono 15 g. hydrate Sodium acetate 20 g.

Water to 1 liter Temperature l75-2(l(t F pH 5 (appro.t.}

Catalyzed cloth was plated by immersion in the above formulation containing stabilizers in amounts and with results as set forth in the following table;

STABILIZING AGENTS (p.p.m.)

Bath Time to Trig- No. Mercury Other ger, (mind 1h phdt1 \|t1lfCllrlC 7-) acetate 30) 37 Lmercaptohenzo- 3-4 thiit'lulc 12) 3X phcnylnicrcuric Z-mercaptuhenzw 2t) acetate (20) thiiwulc (2) 3'9 ntethylhutoxyethanul 2-3 (25) 40 phenylmercuric ntethylbutoxyethanol It) acetate 120 (25) 41 lead acetate (2) 2-3 42 phcnylmercufic lead acetate (3) l acetate (20) 43 thiodiethanol -6 I001 44 phenylmercuric thiodiethanol 19-21 acetate t W0) 45 hutynediol (20} 2-3 46 henylmercuric butynediol (20} l0-l2 47 acetylenedicar- 2-3 boxylic acid (50) 48 merasyl' acctylenedicarhoxl 0- I Z (20) ylic acid (50) 4 potassium acid lZ-IS iodate I5) 50 meroxyl 2 potassium acid 60 (20) iodate tlS) Sodium salt of O-It3-hydrusymercuri-Z-mcthoxypropyll-carhamyl ]-phcnoxyitcetic -phennxyacelic acid Sodium salt of 2,4-tlihydruxy--3 5-dtt hydroxymercurt] hem'uphenone-Z sultonate EXAMPLES 5054 Nickel chloride dihydrate 25 g. Sodium glycolate l5 5. Dimethylaminehoranc 3 g.

\Nater to l liter Temperature l752(l0 F. pH 6 (approx. 1

Catalyzed cloth was plated by immersion in the above solution containing stabilizers in amounts and with results as set forth in the following table:

STABILIZING AGENT(S) (p.p.m.)

acetate tZU) oxyethanol (25) wall deposition after I hr.

EXAMPLE 55 Nickel Chloride dihydrate 1t] 2 Sodium hypophosphite mono- 10 g hydrate Ammonium chloride [00 g. Ammonium hydroxide to pH 8-9 Water to l liter Temperature FIT-200 F The above formulation used to plate catalyzed cloth exhibited heavy sidewall deposition after 1 hour. With the addition of 20 parts per million phenylmercuric acetate to a fresh solution of the same formulation, no sign of solution decomposition was evident after use to plate catalyzed cloth after a period of] hour.

Catalyzed cloth was immersed in the above formulation with the result that hath decomposition occurred in 5 to 6 minutes. With the addition of 60 ppm. phenylmercuric acetate to the formulation, catalyzed cloth was left in the solu tion for a period in excess of 1 hour without bath decomposition.

From the above, it should be readily apparent that the following advantages are gained from the addition of a source of a covalent mercury compound to an electroless metal solution:

I. Covalent mercury compounds used alone are primary stabilizers for electroless metal solutions.

2. Covalent mercury compounds in combination with prior art stabilizers for electroless metal solutions exhibit a synergism that improves stability of solution to an extent greater than that obtainable with either the mercury compound or the prior art stabilizer used alone.

37 The addition of a covalent mercury compound to an electroless metal plating solution does not significantly affect the rate of deposition, as with other stabilizing agents known as catalytic poisons.

4. The addition ofa covalent mercury compound to an electroless metal plating solution allows for a greater operational temperature tolerance in that higher temperatures are permissible due to the stabilizing effect of the mercury compound.

5. The addition ofa covalent mercury compound to an electroless plating solution, to some extent, lessens the detrimental effect of contaminants dragged into the plating bath.

The reason for improved bath stability resulting from the addition of a source of a covalent mercury compound to the electroless metal solution is not fully understood, but is be lieved to be associated with the evolution of hydrogen gas as the metal deposits. An unusual property of mercury is its high overvoltage for hydrogen and it is believed that this property is associated with the stabilization mechanism.

It should of course be understood that changes may be made in the specific embodiments described herein without departing from the scope of the invention as defined by the following claims:

We claim:

1. in an aqueous electroless copper solution comprising a source of cupric ions, a reducing agent therefor, a complexing agent for the cupric ions in an amount sufficient to prevent precipitation of the cupric ions in solution, and a source of hydroxide ions; the improvement comprising a solution soluble covalent mercury compound corresponding to the formula RHgR where R is a covalently bonded organic radical selected from the group of alkyl, cycloalkyl, aryl, alkaryl, aralkyl, alkoxy, aryloxy and heterocyclic radicals and R is the same as R or a polar group selected from the group of NO SO. ,OH and alkali metal salts thereof, COOH and alkali metal salts thereof, NH CN and halide in a small but effective amount capable of providing increased bath stability.

2. The composition of claim 1 where R is a polar group.

3. The composition of claim 2 where R is the sodium salt of SO OH.

4. The composition of claim 1 where the covalent mercury compound is in solution in an amount varying from 1 to 100 parts per million.

5. The composition of claim 1 further characterized by the addition of a second stabilizer for an electroless copper solution.

6. The composition of claim 2 where the second stabilizer is selected from the group consisting of divalent sulfur compounds, cyanide compounds including nitriles and dinitriles and acetylinic compounds.

7. The composition of claim 2 where the acetylinic compound corresponds to one of the following formulas:

RC E CH or RC E CR" where R, R and R" are radicals selected from the group of monovalent hydroxyalkyl, cyclohydroxyalkyl and hydroxyalkyl ether.

8. In an aqueous electroless copper solution comprising a source of cupric ions, a reducing agent therefor, a complexing agent for the cupric ions in an amount sufficient to prevent precipitation of the cupric ions in solution and a source of hydroxide ions; the improvement comprising the addition of phenyl mercuric acetate in a small but effective amount capable of providing increased bath stability.

9. In an aqueous electroless nickel solution comprising a source of nickel ions, a hypophosphite reducing agent therefor, and a pH adjuster; the improvement comprising a solution soluble covalent mercury compound corresponding to the formula RHg-R' where R is a covalently bonded organic radical selected from the group of alkyl, cycloalkyl, aryl, alkaryl, aralkyl, alkoxy, aryloxy and heterocyclic radicals and R is the same as R or a polar group selected from the group of NO SO OH and alkali metal salts thereof, COOH and alkali metal salts thereof, -NH CN and halide in a small but effective amount capable of providing increased bath stability.

10. The composition of claim 9 where R is a polar group.

11. The composition of claim 9 where R is the sodium salt of SO OH.

12. The composition of claim 9 where the covalent mercury compound is in solution in an amount varying from 1 to parts per million parts of solution.

1 The composition of claim 9 including a complexing agent for nickel ions in an amount capable of preventing precipitation of the nickel ions in solution.

14. The composition of claim 7 in combination with a second stabilizer for electroless nickel solutions.

15. The composition of claim 2 where the second stabilizer is selected from the group consisting of divalent sulfur compounds, lead ions, iodate compounds and acetylinic compounds.

16. The composition of claim 3 where the acetylinic compound corresponds to one ofthe following generic formulas:

R-C 5 CH or R'C E CR" where R, R and R" are radicals selected from the group of monovalent hydroxyalkyl,

cyclohydroxyalkyl and hydroxyalkyl ether.

17. The composition of claim 7 where the second stabilizer is lead ions.

18. The composition of claim 7 where the second stabilizer is a thio compound.

19. The composition of claim 7 where the second stabilizer is an iodate compound.

20. In an aqueous electroless nickel solution comprising a source of nickel ions, a hypophosphite reducing agent therefor, and a pH adjuster; the improvement comprising phenylmercuric acetate in solution in a small but effective amount capable of providing increased bath stability. 

2. The composition of claim 1 where R'' is a polar group.
 3. The composition of claim 2 where R'' is the sodium salt of -SO2OH.
 4. The composition of claim 1 where the covalent mercury compound is in solution in an amount varying from 1 to 100 parts per million.
 5. The composition of claim 1 further characterized by the addition of a second stabilizer for an electroless copper solution.
 6. The composition of claim 2 where the second stabilizer is selected from the group consisting of divalent sulfur compounds, cyanide compounds including nitriles and dinitriles and acetylinic compounds.
 7. The composition of claim 2 where the acetylinic compound corresponds tO one of the following formulas: R-C*CH or R''-C*C-R'''' where R, R'' and R'''' are radicals selected from the group of monovalent hydroxyalkyl, cyclohydroxyalkyl and hydroxyalkyl ether.
 8. In an aqueous electroless copper solution comprising a source of cupric ions, a reducing agent therefor, a complexing agent for the cupric ions in an amount sufficient to prevent precipitation of the cupric ions in solution and a source of hydroxide ions; the improvement comprising the addition of phenyl mercuric acetate in a small but effective amount capable of providing increased bath stability.
 9. In an aqueous electroless nickel solution comprising a source of nickel ions, a hypophosphite reducing agent therefor, and a pH adjustor; the improvement comprising a solution soluble covalent mercury compound corresponding to the formula R-Hg-R'' where R is a covalently bonded organic radical selected from the group of alkyl, cycloalkyl, aryl, alkaryl, aralkyl, alkoxy, aryloxy and heterocyclic radicals and R'' is the same as R or a polar group selected from the group of -NO2, -SO2OH and alkali metal salts thereof, -COOH and alkali metal salts thereof, -NH2, -CN and halide in a small but effective amount capable of providing increased bath stability.
 10. The composition of claim 9 where R'' is a polar group.
 11. The composition of claim 9 where R'' is the sodium salt of -SO2OH.
 12. The composition of claim 9 where the covalent mercury compound is in solution in an amount varying from 1 to 100 parts per million parts of solution.
 13. The composition of claim 9 including a complexing agent for nickel ions in an amount capable of preventing precipitation of the nickel ions in solution.
 14. The composition of claim 7 in combination with a second stabilizer for electroless nickel solutions.
 15. The composition of claim 2 where the second stabilizer is selected from the group consisting of divalent sulfur compounds, lead ions, iodate compounds and acetylinic compounds.
 16. The composition of claim 3 where the acetylinic compound corresponds to one of the following generic formulas: R-C*CH or R''-C*C-R'''' where R, R'' and R'''' are radicals selected from the group of monovalent hydroxyalkyl, cyclohydroxyalkyl and hydroxyalkyl ether.
 17. The composition of claim 7 where the second stabilizer is lead ions.
 18. The composition of claim 7 where the second stabilizer is a thio compound.
 19. The composition of claim 7 where the second stabilizer is an iodate compound.
 20. In an aqueous electroless nickel solution comprising a source of nickel ions, a hypophosphite reducing agent therefor, and a pH adjuster; the improvement comprising phenylmercuric acetate in solution in a small but effective amount capable of providing increased bath stability. 